1. Technical Field
The present invention relates to a tire monitor device for monitoring information such as an air pressure and the like of a tire using wireless radio wave.
2. Related Art
Generally, a tire (also referred to as wheel) of a vehicle such as an automobile, a trailer, or an airplane is filled with gas such as air and nitrogen at a defined pressure, but the filled gas changes volume due to travel heat and the like of the tire or gradually decreases due to air leakage and the like. Therefore, in order to satisfactorily maintain traveling stability or to avoid worst cases such as an explosion (burst), a daily checkup such as a checkup using pneumatic gauge and the like is vital.
However, daily checkup can only be carried out while the vehicle is stopped, and furthermore, the daily checkup itself is not performed in many cases, and thus a technique of constantly monitoring the information (air pressure etc.) of the tire even while traveling is desired.
For such a technique, a “tire pressure monitoring system and puncture automatic repair device” disclosed in Japanese Unexamined Patent Publication No. 2006-281977 is known. This technique is hereinafter referred to as a “related art”.
FIG. 10 shows a conceptual view of the related art. In the figure, a vehicle such as an automobile (hereinafter simply referred to as vehicle) 1 is drawn looked down from above, where front, back, left and right are as shown in the figure. The top of the figure is “front”, the bottom is “back”, the left is “left”, and the right is “right”.
The vehicle 1 is a four-wheeled automobile with four tires 2 to 5. That is, the vehicle 1 includes a front left tire 2, a front right tire 3, a back left tire 4, and a back right tire 5. When a “position” of such tires is important, the tires are called the “front left”, “front right”, “back left”, and “back right” tires, but in other cases, the tires are simply referred to as the “tire”.
The tire monitor device of the related art includes four LF antennas 6 to 9 installed near (e.g., in tire house) the corresponding tires 2 to 5 of the vehicle 1, one UHF antenna 10 common to each tire 2 to 5, a control unit 11, and a display unit 12, and also includes a sensor unit (for sensing information such as air pressure and the like of tire, also referred to as transponder), which is not shown, attached to each tire 2 to 5.
Here, “LF” of the LF antennas 6 to 9 is an abbreviation for a wireless signal of low frequency band (e.g., several hundred KHz band), and similarly, “UHF” of the UHF antenna 10 is an abbreviation for wireless signal of high frequency band (e.g., several hundred MHz band).
Similar to each tire 2 to 5, when the “position” of the four LF antennas 6 to 9 is important, the antennas are called a “front left”, “front right”, “back left”, and “back right” antennas, but in other cases, the LF antennas are simply referred to as the “LF antenna”.
The tire monitor device of the related art configured as above operates in the following manner. The sensor unit (not shown) attached to each tire 2 to 5 is normally in a standby state (standby state of response request signal from LF antenna), where when receiving the response request signal from a nearby LF antenna, the sensor unit returns information of the attached tire (generally, information of air pressure, ID number unique to each sensor unit, and the like) with a UHF signal in response to the response request signal.
The control unit 11 performs a control of transmitting the response request signal in time division from each LF antenna 6 to 9 while itinerating through the four LF antennas 6 to 9 in order, and also performs a control of retrieving the response signal from each sensor unit with respect to the response request signal received at the UHF antenna 10 and reproducing information of each tire 2 to 5, and for example, determines whether or not the air pressure is at the defined air pressure, and outputs the determination result to the display unit 12 to notify a passenger of the vehicle 1.
Therefore, according to the related art, if the air pressure of a specific tire such as the front left tire 2 is lower than the defined air pressure, the lack of air pressure of the tire (front left tire 2) is determined in the control unit 11 from the information contained in the response signal transmitted from the sensor unit of the front left tire 2 with respect to the response request signal from the front left LF antenna 6, and the result is notified to the passenger through the display unit 12, so that necessary countermeasures can be immediately taken even during traveling to achieve traveling stability and safety. Other than the above-described related art, for example, the air pressure of the front left tire 2 may be stored in the control unit with respect to the response request signal from the front left LF antenna 6 with the ID number of the sensor unit contained in the response signal transmitted from the sensor unit of the front left tire 2 corresponded with the tire positional information of “front left tire”. That is, the sensor unit ID is stored in a vehicle body side control device. If the ID number and the tire position are corresponded, which sensor unit of the tire position the response signal is from can be identified by identifying the ID number contained in the response signal intermittently transmitted by the sensor unit without transmitting the response request signal from the LF antenna.
The operation of the related art is satisfied based on an assumption that four tires 2 to 5 and four LF antennas 6 to 9 are paired one to one to carry out “response request” and “response”. That is, in order to obtain the operation of the related art, only the sensor unit of the front left tire 2 responds with respect to the response request of the front left LF antenna 6, only the sensor unit of the front right tire 3 responds to the response request of the front right LF antenna 7, only the sensor unit of the back left tire 4 responds to the response request of the back left LF antenna 8, and only the sensor unit of the back right tire 5 responds to the response request of the back right antenna 9.
To this end, the LF antenna and the tire that form a pair need to be brought as close as possible, but an attachment position of the sensor unit to the tire is a predetermined position on an outer periphery of the wheel, or a predetermined position on an inner wall of the tire, where either position is biased in a radial direction of the tire, and thus a distance with the LF antenna becomes closer or distant with the rotation of the tire, whereby a gain and a radiation pattern, transmission power, and the like of the LF antenna need to be suitably adjusted in view of a maximum distant distance with the LF antenna.
However, the vehicle 1 incorporating the tire monitor device is not only one type (one vehicle model), and the shape of the body is different, the size of the tire as well as the shape of the tire house are different, and furthermore, the distance between the tires (distance of front, back, left, and right) is also different, and thus the gain and the radiation pattern, the transmission power and the like of the LF antenna need to be adjusted each time in correspondence to such difference, which obviously takes time and effort.
For instance, assume making an adjustment so that only the sensor unit of the front right tire 3 responds to the response request of the front right LF antenna 7. Specifically, in FIG. 10, a case of adjusting such that only the sensor unit of the front right tire 3 returns a response signal 14 with respect to a response request signal 13 of the front right LF antenna 7 is considered. The purpose of adjustment in this case is to have only the sensor unit of the front right tire 3 to return the response signal 14, in other words, to have other tires not to respond. A characteristic that a magnetic force exponentially attenuates as the distance becomes further apart in a free space is provided. If metal such as a vehicle exists, the magnetic force is transmitted along a metal surface, and thus reception is facilitated at the sensor unit attached to the front left tire 2, which is not desired (see reference numeral 13a). A so-called crosstalk then easily occurs.
In this case, the gain or the radiation pattern, the transmission power, and the like of the front right LF antenna 7 are adjusted such that only the sensor unit of the front right tire 3 returns the response signal 14, that is, the sensor unit of the front left tire 2 does not return a response signal 15. In performing such adjustment, the sensor unit of which tire is responding cannot be determined and thus adjustment is difficult in the related art.
For example, whether the sensor unit of the front right tire 3 is returning the response signal 14, or whether the sensor unit of the front left tire 2 is returning the response signal 15 with respect to the response request signal 13 of the right left LF antenna 7 cannot be determined. Thus, in the related art, the adjustment of the LF antenna can only be carried out through a trial and error method, whereby extra trouble and time are required by such amount.